LCoS display system with software/firmware correction enabling use of imperfect LCoS chips

ABSTRACT

A LCoS projection display system using two LCoS chips and software or firmware to control the chips so that any defects in the first chip that result in incorrectly displayed pixels are corrected by production of compensating illumination provided by the second LCoS chip.

FIELD OF THE INVENTIONS

The inventions described below relate the field of LCoS displays.

BACKGROUND OF THE INVENTIONS

Projection displays for large screen televisions and monitors may useLCoS (Liquid Crystal on Silicon) chips to create an image. The image iscreated by directing light from a powerful lamp onto the surface of aLCoS chip, and operating the LCoS chip to control, with very fineresolution, the polarization of the reflected light, and passing thereflected light through a polarizer onto a display screen. At each pixelof the LCoS chip, the polarization of the reflected light may be changeddepending on the state of the pixel. The light reflected by the LCoSchip is directed into a polarizer, and light of proper polarization ispassed through a polarizer the screen, while light of improperpolarization is reflected away from the screen, thereby differentiatingthe light and dark pixels in the image. The reflected light then passesthrough a projection lens to a screen.

A typical LCoS Projector of the prior is illustrated in FIG. 1, whichillustrates in schematic form a traditional projector setup 1 utilizingone LCoS chip to create a projected image. In this system, a lamp andreflector assembly 2 provide the powerful light source, producing anddirecting light beams 3 into integrating rod 4, where light beams 3 arehomogenized. Light beams 3 then exit the integrating rod and passthrough color wheel 5. Color wheel 5 is rotated by a motor, which isgoverned by electronics, both not illustrated. The color wheel isthereby controlled so that it causes light beams 3 to pass through aspecific color filter at an instant in time, therefore filtering only aspecific color of light at that time. Light beams 3, now a certainuniform color (typically red, green or blue), next pass through optics6.

Optics 6 focuses light beams 3 on polarizing beam splitter 7. Polarizingbeam splitter 7 is positioned such that the polarizing surface is at aforty-five degree angle to the optical axis of optics 6. The polarizingbeam splitter is also positioned at a forty-five degree angle to displaysurface 8 of LCoS chip 9. Polarizing beam splitter 7 divides light beamsinto two components: beams of light that have S polarization and beamsof light that have P polarization. The beam splitter does this byreflecting the beams of light that have the opposite polarization thandoes the polarizing beam splitter and allowing the beams of light thathave the same polarization as the polarizing beam splitter to passthrough the polarizing beam splitter unaffected. For example, ifpolarizing beam splitter 7 is an S polarizing beam splitter, then lightbeams 3 are divided into light beams 10 having P polarization and lightbeams 11 having S polarization. Light beams 10 reflect off polarizingbeam splitter 7 at roughly a ninety degree angle to light beams 3, andproject onto LCoS chip 9, while light beams 11 pass through polarizingbeam splitter 7. Light beams 11 may either be lost (not used to displayan image on a screen), or may be projected back onto LCoS chip 9 throughthe use of polarization recovery optics (not illustrated).

LCoS chip 9 is controlled by electronics (not illustrated) that governthe state of the pixels on display surface 8 of the LCoS chip. Theelectronics control the image displayed on display surface 8 byselectively turning pixels on the display surface “on” and “off”. IfLCoS chip 9 is a monochromatic display (capable of only displaying blackor white), the pixels on display surface 8 can only be in an “on” or“off” state. If the chip is not a monochromatic display, that is, thechip can display grayscale, then the pixels on display surface 8 canoccupy states between the fully “on” and fully “off” positions.

Light beams 10 then reflect off LCoS chip 9 at a roughly 180 degreeangle to the incident light beams, and the state of the polarization ofthese light beams (originally P polarization) depends on the state ofthe pixels on display surface 8. For example, LCoS chip 9 may bedesigned such that light beams that reflect off pixels that are “on”have their polarization state rotated by ninety degrees. Light beamsthat strike “off” pixels reflect with their polarization stateunaffected. Thus, light from light beams 10 that strikes “on” pixels isreflected back from display surface 8 with its polarization rotated fromS to P. These light beams, since they now have the same polarization asthe polarizing beam splitter, pass through the polarizing beam splitterto projection optics 12, which focus light beams 13 onto display screen14 (not illustrated). Optics 12 are positioned such that the opticalaxis of the optics is perpendicular to display surface 8 of LCoS chip 9.Light from light beams 10 that strikes “off” pixels is reflected backfrom LCoS chip 9 with its polarization unchanged. When these light beams15 strike polarizing beam splitter 7, the light beams reflect off thepolarizing beam splitter at a roughly ninety degree angle to theincident light beams and pass back through optics 6. If LCoS chip 9 is agrayscale-capable chip, then light from light beams 10 may also reflectoff partially “on” pixels, and thus undergo a polarization rotationbetween zero and ninety degrees. The light reflected from partially “on”pixels is partly transmitted through polarizing beam splitter 7 andpartly reflected by it.

Through this pixel and light polarization manipulation, images on theLCoS chip are projected onto the screen. “On” pixels in the LCoS chip'simage reflect light that passes through the polarizing beam splitter,and thus create a bright area in the projected image. “Off” pixels inthe LCoS chip's image reflect light that the polarizing beam splitterblocks from reaching the display screen, creating a dark area in theprojected image. Light that reflects off partially “on” pixels is usedto create shades between the two extremes; the degree of how “on” thepixel is determines how light the shade is.

Images displayed by LCoS chips are magnified around one hundred times(100×) when they are projected on to a display screen. Therefore, anyproblems with defect pixels on the LCoS chip are going to be noticeableand annoying to a projector viewer. During the life of the system, it ispossible that process defects in the LCoS chip may cause visual defectsto appear in the projected image. Pixels may lose the ability to switchcompletely on and off before a display has reached the end of itslifetime. Some LCoS chips have defects after manufacturing which are notrelated to lifetime and are simply defects on the chip. Defects may leadto failure of pixels to operate. If a pixel is stuck in a permanent “on”or “off” state, it will appear on the screen as a permanent, non-movingwhite or black spot, respectively. These defects will cause theprojected image to have artifacts that are not part of the intendedimage. This degrades image quality and is undesirable in an end product.FIGS. 2 a and 2 b illustrate this degradation of the image displayed tothe viewer. In FIG. 2 a, the display screen 14 shows an image whichincludes a black dog, but due to a single pixel on the LCoS chip whichis stuck permanently on, a single pixel that should be black is insteadilluminated, creating a noticeable visual artifact 16 on the image. InFIG. 2 b, the display screen 14 shows an image which includes a whitedog, but due to a pixel on the LCoS chip which is stuck permanently off,a single pixel that should be white is not illuminated, creating anoticeable visual artifact 17 on the image. These artifacts appearpermanently on the display screen because they derive from physicaldefects in the LCoS chips. Systems that utilize only one LCoS chip toproduce an image or portion of an image have no way to correct orameliorate the effects that bad, non-functioning pixels have on animage.

SUMMARY

The systems and methods described below mitigate the impact of blemishesand pixel defects in LCoS projection systems. Given that the majority ofdefects impact only a small area of the display, the system uses asecond LCoS chip so that defects on one chip can be hiddenalgorithmically by the way in which the image is presented on the other.For example, if there is a pixel which is stuck “on” in one LCoS chip,then the system can mitigate its effect by turning off that area in theother LCoS. Note that perfect correspondence between pixels on one LCoSand the pixels on the other LCoS (‘pixel-alignment’) is not necessary toachieve this effect. The system and method have the added benefit ofallowing the use of non-zero-defect LCoS display chips. It also obviatesthe need for expensive polarization recovery optics in order to utilizethe entire projected lamp light.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a typical LCoS Projector of the prior art, asdiscussed above.

FIGS. 2 a and 2 b illustrates the image artifact which the systemaddresses.

FIG. 3 is a schematic of a LCoS projector system using two LCoS chips toprocess a single image frame. FIG. 4 illustrates the compositing ofimages from each LCoS chip to compensate for a defect in one of thechips.

FIG. 5 illustrates the compositing of images from each LCoS chip tocompensate for a defect in one of the chips.

FIG. 6 illustrates the expected arrangements of nearby pixels from eachLCoS chip.

FIG. 7 illustrates the expected arrangements of nearby pixels from eachLCoS chip.

DETAILED DESCRIPTION OF THE INVENTIONS

FIG. 3 is a schematic of a LCoS projector system 20 using two LCoS chipsto process a single image frame. As in FIG. 1, the system includes thelight source 2, integrator rod 4, color wheel 5, optics 6 which providethe initial light beams 3 to the remainder of the system, and the beamsplitter 7 which controls redirection and passage of light to thevarious surfaces of the system. The system may be part of awall-projector, a projection television, a monitor or the like.

The light source comprises a typical lamp and reflector assembly. Atypical configuration with a UHP lamp is shown, though any othersuitable light source may be used, such as LED assemblies or halogenlamps. The light provided by the light source is not polarized and,generally, not homogenized. The integrating rod serves to homogenize thelight provided by the light source. The variable color filter orsequential color displays may be a typical color wheel or anelectronically switchable spectral filter (ColorLink's Colorswitch® forexample) or similar device capable of presenting color filters in thelight path in a sequence correlated to the operation of the LCoS chipsurface. The beam splitter is a polarizing beam splitter cube orreflective polarizer. This optical component (either a prism withspecial coatings on the hypotenuse or a wire grid type polarizing beamsplitter) passes one state of linear polarization and reflects theother. Thus the incoming light from the lamp is split into its S and Pcomponents by this beam splitter. For the purposes of illustrationassume that this polarizing beam splitter is S polarized.

The beam splitter provides light to both the first LCoS chip 9 and asecond LCoS chip 21. The beam splitter reflects light of onepolarization state orthogonally while permitting light of anotherpolarization state to pass through. Though the beam splitter may reflectlight of S or P polarity and pass the other, and either arrangement issuitable.

Each LCoS chip is controlled by a control system, which may beimplemented in firmware or software which are programmed to interpretincoming image display data and generate corresponding appropriatecontrol signals to the millions of pixels on the LCoS chip. The imagedata may include any typical signal, such as broadcast televisionsignal, computer display image data, cable video signals or videoequipment output. The electronics control the image displayed on displaysurfaces 8 and 22 to correspond to the image display data and thedesired image display light output from the chip. The LCoS chips arepositioned at 90° angles from each other, with the beam splitterdisposed such that its reflecting surface is angled 45° from each chip.Referring to the first LCoS chip 9, the beam splitter reflects thatportion of the incoming light beam 3 which matches its polarity,resulting in polarized light beam 10 p directed toward LCoS 9 and itsdisplay surface 8. The reflected polarized light beam 13 reflected bythe beam splitter has the sense of the polarization changed, or not,based on the state of the pixel upon which it is incident. Forsimplicity, we will assume that the light is rotated by 90 degrees or 0degrees depending on pixel state. Thus, light reflecting which has beenrotated by 90 degrees is now polarized in such a way that it will passthrough the reflective polarizer 7 and pass through the beam splitter aslight beam 13 s. This light goes on to exit the system throughprojection optics 12. Light which was not changed by the first LCoS 9reflects and goes back towards optics 6, as is represented by light beam13 p.

Referring to the second LCoS chip 21, the beam splitter transmits thatportion of the incoming light beam 3 which matches its polarity,resulting in polarized light beam 11 s directed toward the second LCoS21 and its display surface 22. This light beam 11 s has the sense of itspolarization changed, or not, based on the state of the pixel in thesecond LCoS upon which it is incident, and is reflected in light beam23, which includes some P polarized light and some S polarized light.Light reflecting which has been rotated by 90 degrees is now polarizedin such a way that it will not pass through the reflective polarizer 7is reflected as light beam 23 p toward the projections optics 12. Lightreflecting which has not been rotated is still polarized such that itwill pass through the reflective polarizer is represented by light beam23 s.

The LCoS chips, polarizing beam splitter, and optics may be positionedsuch that light reflecting off of a pixel on display surface 8 perfectlycoincides, after going through the polarizing beam splitter andprojection optics, with the light reflecting off a pixel in the exactsame location on display surface 22 after that light reflects off thepolarizing beam splitter and travels through the projection optics (thisis referred to hereafter as ‘pixel-alignment’). The LCoS chips,polarizing beam splitter, and optics may also be positioned so thatlight reflecting off corresponding pixels on display surfaces 22 and 8does not perfectly coincide (non-pixel-aligned).

The polarization state of the light beams reflected by the LCoS chips iscontrolled by the state of the pixels on the chips' display surfaces.Electronics control the state of the pixels that produce images on theLCoS chip display surfaces by selectively turning pixels on the displaysurface “on” and “off”. If the chips are monochromatic displays, thepixels on display surfaces can only be in an “on” or “off” state; if thedisplays are not monochromatic, then the pixels on display surfaces canoccupy states between the fully “on” and fully “off” positions. The LCoSchips may simultaneously display the same image, or they may displayimages that differ slightly from, as will be explained below. If lightfrom light beams 10 or 11 strikes an “on” pixel in either of the LCoSchip display surfaces, then the polarization of the reflected light isrotated ninety degrees, to “S” or “P”, depending on the originalpolarization state. If light from the light beams strikes an “off” pixelin either of the LCoS chips, then the polarization of the reflectedlight remains the same. The polarization state of the light beamsreflected by an LCoS chip-determines whether or not the reflected lightemits into the projection optics or if the light is blocked fromreaching the projection optics. If light from either light beams 10 or11 strike “off” pixels, then reflected light beams 13 and 23(respectively) have the same polarization state as the incident lightbeams. In the case of reflected light beam 23, if it reflects off an“off” pixel on LCoS chip 21, then it retains the “S” polarization state.Light beam 23 p passes through polarizing beam splitter 7 and out tooptics 6, thereby not reaching the projection optics 12. It iseffectively blocked from projection optics 12. If reflected light beam13 reflects off an “off” pixel on LCoS chip 9, then it has a “P”polarization state. Light beam p reflects off polarizing beam splitter 7at a roughly ninety degree angle to light beam 13 and travels out tooptics 6, not transmitting to projection optics 12.

Light from light beams 10 and 11 may also reflect off partially onpixels, and thus undergo a polarization rotation between zero and ninetydegrees. The light reflected from partially on pixels is partlytransmitted through polarizing beam splitter 7 and partly reflected byit, so a fraction of the light is able to transmit to the projectionoptics.

Through this pixel and light polarization manipulation, images on theLCoS chips are projected onto the display screen. “On” pixels in theLCoS chips' images reflect light that the polarizing beam splitterdirects to the projection optics, and thus creates a bright area in theprojected image. “Off” pixels in the LCoS chips' images reflect lightthat the polarizing beam splitter directs perpendicular to theprojection optics, blocking it from the screen, and therefore creating adark area in the projected image. Light that reflects off partially“on”, pixels is used to create shades between the two extremes. Thedegree of how “on” the pixel is determines how light the shade is, assome of the light reflected by a partially “on” pixel is directed to theprojection optics, and some is directed away from the projection optics.

Because only half of the lamp's light output reflects off a given LCoSchip, the projected images from each LCoS chip are only half as brightas an image projecting from a single LCoS chip projector that uses theentire available lamp light. The two images from the dual LCoS chipprojector are combined in the same, or very nearly the same, space onthe image display screen, however, so the resulting image from dual LCoSchip projector 20 is as bright as an image from a single LCoS chipprojector, assuming that both projectors use similar lamps, optics, andreflector systems.

Thus, each pixel of illumination on display screen 14 may be illuminatedby two beams of light. Thus, the system may be operated to ensure thatevery pixel of the display screen is illuminated, when appropriate, byat least one of the LCoS chips. This mitigates the effect of a pixelstuck in the non-transmissive (relative to the beam splitter) state onthe first LCoS chip. Thus, the advantage of using two LCoS chips todisplay identical or nearly identical images on to the same imagingscreen is that allows for the potential to eliminate or greatly reducethe loss of image quality caused by defect pixels that are perpetuallystuck “on” or “off”. Therefore, it is not necessary to build the twoLCoS chip display with zero-defect LCoS chips. Also, if a problem withdefect pixels on one or both of the LCoS chips arises during thelifetime of the projector, then the projector can be re-programmed torestore projector image quality to nearly original specifications. Forexample, if the location of a defect pixel on a first LCoS chip isknown, then the electronics controlling the second LCoS chip cancompensate by increasing or decreasing the brightness of thecorresponding pixel on the second LCoS chip (if the LCoS chips arepixel-aligned) or pixels on the second LCoS in the region of the defectpixel on the first LCoS chip. The overlay of a defect pixel with acompensated pixel or pixels renders a combined portion of an image thatmore accurately matches the intended image. The images from the two LCoSchips are combined by the projection optics so that a viewer watchingthe imaging screen can only discern one image.

Another advantage of the two LCoS chip projector is that costlypolarization recovery optics are not necessary to utilize all theemitted light from the lamp and reflector. Normally these optics arenecessary to capture the light that has the same polarization as thepolarizing beam splitter and is thus not reflected on to the LCoS chip.Alternatively, if the light output from a single LCoS is sufficient incertain applications, the second LCoS chip may be operated solely toprovide compensating illumination in areas which are over or underilluminated by the firth LCoS chip.

The device described above may be embodied in microdisplay LCoSprojectors that use three microdisplays (one reflecting red light, onereflecting green light, and one reflecting blue light) and red, green,and blue lamps instead of the single microdisplay and color wheel. Inthis system, each single color LCoS microdisplay would be replaced witha dual chip LCoS microdisplays described above. Only microdisplayswithin a particular microdisplay pair (red light, green light, or bluelight) would compensate for defect pixels within that pair. For example,microdisplays in the green light microdisplay pair just compensate fordefect pixels in the green light microdisplay pair; a green lightmicrodisplay is not used to correct for a defect in a blue lightmicrodisplay. The total output of each single-color dual-chip LCoSdisplay is combined for illumination of the display screen so that theuser sees the proper blended image. Nonetheless, where necessary, thesystem could be operated such that an overly bright light colored pixelcould be painted over in a darker color or combination of colors fromthe remaining color sources.

Both of the defect pixel related image-display problems illustrated inFIGS. 2 a and 2 b (i.e., the over-illumination of under-illumination ofa display screen pixel area) can be corrected or at least partiallycorrected by adjusting the images displayed by the LCoS chips. Forexample, with reference to FIG. 4 and continuing reference to FIG. 2,suppose that LCoS chip 9 in dual LCoS chip projector 20 has a knowndefect pixel that is permanently “on” in the center of display surface8. In this example assume LCoS chip 9 and LCoS chip 21 arepixel-aligned, meaning that the chips are mechanically aligned such thatparticular pixels of each chip transmit light to a particular pixel ofthe display screen. The ideal image dual LCoS chip projector 20 attemptsto display is a rectangle that is a 50% gray shade, i.e. a gray exactlyhalfway between 100% black and 100% white. If LCoS chip 9 and LCoS chip21 were both defect-free, therefore, they would each display a 50% grayrectangle that is half the brightness of the intended final image. Dueto the defect pixel on display surface 8, however, LCoS chip 9 can onlygenerate an image that, when projected, looks like image 50, with andunintended bright spot 52. This spot is fully white because all thelight reflecting off the defect pixel is projecting on to the imagedisplay screen. Were LCoS chip 9 used in a single LCoS chip projector,this bright spot in image 50 would be noticeable on the image displayscreen. In order to compensate for bright spot 52, the electronics thatdrive LCoS chip 20 switch the corresponding pixel on display surface 22completely “off”, causing LCoS chip 21 to display an image that, whenprojected, looks like image 60, with the intentionally completely darkspot 62. Dark spot 62 is 100% black because all the light reflecting offthat pixel is being blocked from the image display screen. By projectingboth image 60 and image 50 on to the image display screen at the sametime, image 70 results. By mixing bright spot 52 with dark spot 62, 50%gray results at spot 72, which is the intended image shade. A viewerwould be unable to discern any loss in image quality in this case. For apixel-aligned dual LCoS chip projector, full shade compensation canoccur for 50% of the shade spectrum assuming a one-to-one pixelcompensation system is utilized. In this particular scenario, LCoS chip21 can fully correct for any intended shade brighter than 50% gray, suchas 100% white, 25% gray, etc. In this example, the algorithm employed bythe LCoS chip 21 electronics simply computes what color to average 100%white with in order to derive the intended, final shade, or a shade thatbest approximates the desired shad if it is not possible to generate it.If, on the other hand, LCoS chip 9 has a permanently “off” pixel, thenLCoS chip 21 can fully correct for shades between 100% black and 50%gray by lightening the corresponding pixel on display surface 22 fromthe intended, projected shade. Note that in all these examples it isgrayscale gradients that are referred to and not color, as the LCoSchips control grayscale and color is adjusted by color wheel 5. Thus,the control system is programmed to operate the first LCoS chip and thesecond LCoS chip such that each LCoS chip generally provides half theillumination for the brightness of the display indicated in the imagedata, and for each faulty pixel of one LCoS chip resulting in over orunder illumination on the screen display, the control system controlsthe other LCoS chip to provide compensating lesser or additionalillumination from a pixel (in a pixel aligned system) of group of pixels(in a non-pixel aligned system) on other LCoS that correspond to thefaulty pixel.

In certain image situations a pixel-aligned dual LCoS chip projectorcannot fully correct defect-pixel related issues, but it can partiallycompensate them. With reference to FIG. 5 and continuing reference toFIG. 3, again assume that LCoS chip 9 has a known pixel defect in thecenter of display surface 8 and that LCoS chip 9 and LCoS chip 21 arepixel-aligned. The defect pixel on display surface 8 is againpermanently stuck in the “on” position, resulting in a bright, 100%white spot on any image projected by this chip. If dual LCoS chipprojector 20 intends to project an 80% gray rectangular image, LCoS chip9 would display an image that when projected looks like image 80, withbright spot 82. The electronics that control LCoS chip 21 cannot producea shade dark enough in the corresponding pixel on display surface 22 toaverage out to the intended 80% gray; the darkest shade the two pixelscan average to is a 50% gray (100% white averaged with 100% black). Theelectronics that control LCoS chip 21 compensate the maximum possibleand cause the corresponding pixel on LCoS chip 21 to switch completely“off”. This results in an image that when projected looks like image 90,with dark spot 92. When image 80 and image 90 are projected together, itresults in image 100. This image has light spot 102, which is anoticeably lighter shade of gray than the rest of the image. However,this spot is less noticeable than the 100% white spot produced by thedefect pixel on LCoS chip 9. In a situation where full pixel-correctionis not possible, the algorithm controlling the LCoS chip electronicsadjusts the shade of the corresponding pixel in order to bestapproximate the intended display shade.

Programming the electronics that control the LCoS chips in order tocompensate or correct for defect pixels on one or both of the LCoS chipsmay be done as part of a quality control step when a dual LCoS chipprojector is manufactured, as part of routine maintenance of theprojector, or the like. The LCoS chips' electronics programming may beperformed by technician, or it may be accomplished by automatedelectronic program.

The above scenarios all discuss pairing an LCoS chip with one defectpixel with an LCoS chip with zero defect pixels, although it is possibleto pair an LCoS chip with multiple defect pixels with a zero defect LCoSchip, to pair two LCoS chips with multiple defect pixels, and so forth.As long as there are no corresponding defect pixels on the two LCoSchips, each LCoS chip's electronics can be programmed to compensate forthe other LCoS chip's defect pixels.

Although the previous examples all deal with two pixel-aligned LCoSchips, it is possible to achieve similar pixel-defect compensation witha non-pixel-aligned projector. In this case, light reflecting off apixel on one LCoS chip might overlap, when projected on to an imagedisplay screen, with light from two or four pixels from the other LCoSchip. In the case where two pixels from one LCoS chip overlap one pixelfrom the other LCoS chip, the misalignment might be in either thevertical or horizontal direction. FIG. 6 illustrates the expectedarrangements of nearby pixels from each LCoS chip. If four pixels fromone LCoS chip overlap one pixel from the other LCoS chip, then there ismisalignment in both the vertical and horizontal direction. FIG. 6 showsrepresentative projected images for all the cases of pixel misalignment;pixel group 104 illustrates vertical misalignment, pixel group 106illustrates horizontal misalignment, and pixel group 108 illustratesvertical and horizontal misalignment. If the images displayed by any ofthese misaligned LCoS chips were completely static, the overlay of thetwo projected images would look shifted and fuzzy. Because of the rapidsampling and refresh rate of LCoS chips, however, the overlay of theslightly shifted images makes the combined, projected images look fairlysharp and defined.

Compensating a non-pixel-aligned dual LCoS chip projector is similar infunction to compensating a pixel-aligned dual LCoS chip projector. Inthis case, however, one or more pixels may be used to compensate for adefect pixel. Preferably, more than one pixel is used to compensate fora defect pixel, because a greater number of pixels are available tocompensate for a defect pixel, and so a greater degree of ideal imageapproximation can be achieved. For example, with reference to FIG. 7,suppose a dual LCoS chip projector with vertical and horizontalmisalignment is provided. One of the LCoS chips has a known defect pixelin the center of the display surface. The defect pixel is permanently“on”, causing images projected from this LCoS chip to contain a fullywhite spot in the center of the image. If the dual LCoS chip projectorintends to project an 80% gray square image, the LCoS chip with a defectpixel would display an image like image 110, with bright spot 112. Theelectronics that control the non-defective LCoS chip compensate byswitching completely “off” the four pixels that, when projected, overlapthe defect pixel from the first LCoS chip. The second LCoS chip displaysan image that looks like pixel group 120, with dark spot 122 from thefour “off” pixels. When pixel group 110 and 120 are projected together,it results in pixel group 130. This image pixel group 130 appears as alight area, which bordered on four sides by dark stripes from dark spot122, but when averaged out (due to sampling and the like) would looklike pixel group 140. A perfectly corrected image is provided. Thealgorithm for this particular scenario computes which color the fouroverlapping pixels need to be in order to average out the 100% whitedefect pixel. Using this technique, where four pixels are used tocompensate for one defect pixel, fully compensated shades as dark as 80%gray (or as light as 20% gray, if the defect pixel is permanently “off”)can be achieved, versus only 50% for a single-pixel compensated system.

While the preceding examples of pixel-level compensation have all dealtwith image-quality problems that stem from defect pixels, thecompensation method discussed in this paper can be further expanded tocover cases where dirt, dust, and misalignment over time can causepermanently dark or light spots to form on the projected image.

This methods described above can be extended to cover other pixel-basedmicrodisplay projector systems, such as high temperature polysilicon(HTPS of TFT) or micro-electro mechanical systems (MEMs) projectorsystems including DLP (digital light processing) projector systems, orD-ILA projector systems, with each such system adapted to uses multiplemicrodisplays to display redundant images for a particular region of thechromatic scale. For example, two or more gray-scale MEMS microdisplayswith defect pixels could be used in a projector system thattraditionally uses a single gray-scale MEMS microdisplay with a colorwheel, or three pairs of red-green-blue specific HTPS microdisplays withdefect pixels could be used in place of three single, red-green-bluespecific HTPS microdisplays in an HTPS projector system. In these cases,as with the methods described above, the microdisplays may be eitherpixel-aligned or non-pixel-aligned depending on the requirements of theprojector system and the quantity of defects present on themicrodisplays.

Thus, while the preferred embodiments of the devices and methods havebeen described in reference to the environment in which they weredeveloped, they are merely illustrative of the principles of theinventions. Other embodiments and configurations may be devised withoutdeparting from the spirit of the inventions and the scope of theappended claims.

1. A microdisplay projection system comprising: a light source providinglight input to a first input of a beam splitter, said beam splitterbeing functional to reflect a portion of the light input along a firstaxis and transmit a portion of the light input along a second axis, saidbeam splitter also being functional to selectively reflect lightincoming along the first axis or transmit light incoming along the firstaxis to an output of the beam splitter and to selective reflect lightincoming along the second axis to an output of the beam splitter ortransmit light incoming along the second axis; a display screen disposedalong the first axis, a first LCoS chip disposed along the first axis,said first LCoS chip having a surface comprising a plurality of pixelsbeing operable to reflect incident light and selectively change thepolarity of the reflected incident, said first LCoS chip aligned toreflect incident light back along the first axis, whereby a portion ofsaid light will be transmitted along the first axis toward the displayscreen and a portion of said light will be reflected and lost; a secondLCoS chip disposed along the second axis, said second LCoS chip having asurface comprising a plurality of pixels being operable to reflectincident light and selectively change the polarity of the reflectedincident, said second LCoS chip aligned to reflect incident light backalong the second axis, whereby a portion of said light will betransmitted along the second axis and lost and a portion of said lightwill be reflected along the first axis toward the display screen; acontrol system for controlling the operation of first LCoS chip and thesecond LCoS chip to alter the state of pixels thereon to selectivelychange the polarity of incident light reflected from the pixels tocreate an image on the display screen which corresponds to image dataprovided to the control system; said control system being operable tocontrol one of the LCoS chips to selectively operate pixels to providecompensating illumination to the display screen to compensate for adefective pixel on the other LCoS chip.
 2. The microdisplay projectionsystem of claim 1 wherein: the control system is programmed to operatethe first LCoS chip and the second LCoS chip such that each LCoS chipgenerally provides half the illumination for the brightness of thedisplay indicated in the image data, and for each faulty pixel of oneLCoS resulting in over or under illumination on the screen display, tocontrol the other LCoS display to provide compensating lesser oradditional illumination from a pixel of group of pixels on said otherLCoS that correspond to the faulty pixel.
 3. The microdisplay projectionsystem of claim 1 wherein: the control system is programmed to operatethe first LCoS chip and the second LCoS chip such that one LCoS chipsprovides substantially all the illumination for the brightness of thedisplay indicated in the image data, and for each faulty pixel of thatLCoS resulting in over or under illumination on the screen display, tocontrol the other LCoS display to provide compensating additionalillumination from a pixel or group of pixels on said other LCoS thatcorrespond to the faulty pixel.